1. Field of the Invention
This invention relates to a positive temperature coefficient (PTC) polymer composition and a circuit protection device made therefrom, more particularly to a PTC polymer composition containing a voltage resistance-enhancing agent and a polymer stabilizer.
2. Description of the Related Art
U.S. Pat. No. 6,238,598 discloses a PTC polymer composition that comprises a crystalline grafted polymer and a crystalline non-grafted polymer. Addition of the grafted polymer in the polymer composition improves the properties, such as peel strength, low contact resistance, low initial resistance, high trip current, and high peak volume resistance, of a PTC component made therefrom.
U.S. Pat. No. 6,359,053 discloses a PTC polymer composition that comprises a crystalline grafted polymer, a crystalline non-grafted polymer, and an ionomer of an ionic copolymer of the crystalline non-grafted polymer and an ionized unsaturated carboxylic acid. Addition of the ionic copolymer in the polymer composition improves mechanical properties, such as toughness, good low temperature toughness, high impact strength, and high elasticity, of a PTC component made therefrom.
Circuit protection devices, such as a circuit protection device, made from the aforesaid conventional PTC polymer compositions normally have a low voltage resistance. For instance, a circuit protection device made from the a fore said conventional PTC polymer compositions, which has a volume resistivity of less than 50 ohm-cm and which is used in applications that operate at about 10-40 volts, normally has a maximum voltage rating at about 60 volts, i.e., the circuit protection device will likely break or burn out when the applied voltage reaches the maximum voltage resistance. Therefore, there is a need to increase the voltage resistance of the aforesaid conventional PTC polymer compositions without sacrificing other properties of the circuit protection device.
Commercial polymeric PTC heater devices are made from polymer compositions that have a volume resistivity greater than 50 ohm-cm and often in a range of from 200-1000 ohm-cm. Such heater devices normally operate at a high voltage condition, e.g., 110-240 VAC or higher (above 600VAC) . As such, such polymer compositions have a relatively high voltage resistance and have no need for further enhancement required by those for the circuit protection device.
In co-pending U.S. patent application Ser. No. 10/435,065 filed on May 8, 2003, the applicant disclosed a PTC polymer composition that comprises a polymer mixture, a conductive particulate material, and a voltage resistance-enhancing agent which comprises a particulate metal oxide material. Although, by virtue of the voltage resistance-enhancing agent, the voltage resistance of the PTC polymer composition can be significantly increased, there is still a need to improve the life cycle of the same for high voltage applications.
The entire disclosures of the aforementioned patents are incorporated herein by reference.